1. Field of the Invention
The present invention relates to an image display panel such as an extremely large optical display panel for outdoor use.
2. Description of the Prior Art
Any conventional large-dimensional optical display panel installed in a baseball stadium or a soccer stadium uses monochromatic cathode ray tubes each having a spherical surface. Since the spherical surface does not cause external light to reflect from it, viewers can easily watch images on the screen. However, as shown in the publication of the Japanese Patent Application Laid-Open No. 57-208045 (1982), recently, demand has significantly grown for multiple-color cathode ray tubes each having a planar surface for the purpose of increasing vividness of the screen image. When external light reflects from the planar surface or the cathode ray tubes partially discontinue illumination, since the illumination elements are substantially white, the screen image becomes whitish. This makes the image uncomfortable to watch. To improve the contrast of the entire image, the illumination elements themselves or the surface of multiple-color cathode ray tube need to be colored. Nevertheless, to properly color these, there are a variety of technical problems to solve, which are described below.
Coloration of illumination elements themselves does not generate sufficient volume of light. Furthermore, an annealing process is applied to illumination elements which subjects them to a minimum of 400.degree. C. After stabilizing them on a substrate, temperature no material used for coloring in red, blue and green can stand such high annealing temperature. When coloring the planar surface of a multiple-color cathode ray tube with a single colorant, since each illumination element has a specific permeable wavelength different from each other, luminance intensity of the entire screen image decreases, and as a result, viewers cannot distinctly watch the image from a distance.
The volume of light emitted from red, blue, and green luminous elements becomes uneven by presence of those colorants on the surface of multiple-color cathode ray tubes. This prevents the image from generating brilliance.